ath79: set 2048B ECC size for Mikrotik boards using soft ECC

ath79: set 2048B ECC size for Mikrotik boards using soft ECC

Two Mikrotik board families (SXT 5nD R2 and Routerboard 92x are using
software ECC on NAND. Some of them use chips capable of subpage write,
others do not - within the same family, and a common block size is
required for UBI, to avoid mounting errors. Set the ECC step size
explicitly for them to 2048B, so UBI can mount existing volumes without
problems, at the same time allowing to unlocking subpage write functionality,
reuqired for Meraki MR18.

Fixes: 6561ca1fa51 ("ath79: ar934x: fix mounting issues if subpage is not supported")
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>